The present invention relates to fasteners, and more particularly to a fastener including a bolt and a threaded swageable collar.
A type of fastener widely utilized in the aerospace industry typically comprises a bolt having a head, shank and pin tail portions. The shank portion extends through aligned apertures formed in adjacent panels, wherein the shank portion typically includes a single gripping ledge formed thereon. A collar is swaged onto the shank portion in a manner such that the collar is secured to the gripping ledge in place against a surface of one of the panels.
The male fastener, also called a xe2x80x9cstudxe2x80x9d or xe2x80x9cboltxe2x80x9d, is designed to be reusable such that the swaged-on collar the female part of this fastening system is removed by using a special tool. The removal process implies destroying the swaged-on collar by a tool having two jaws that forces the collar end to break open. During the removal process the single gripping ledge on the male fastener could be damaged thus preventing the fastener stud from being reused. The existing conventional fastener using a swaged-on collar was improved by the present applicant as shown in U.S. Pat. Nos. 5,669,746 and 6,019,559 which disclose a fastener comprising a head portion, a flange portion, a shank portion which extends through aligned apertures formed in adjacent panels, and a collar swaged on the shank portion of the fastener. The shank portion comprises a plurality of gripping surfaces having angled surfaces formed between a plurality of spiral grooves. The female part of this fastening system, named xe2x80x9ccollarxe2x80x9d is first positioned over the end of the shank portion and an installation tool then swages the collar thereon. The collar is tightly held by multiple gripping surfaces, thereby preventing movement in the direction of the longitudinal axis of the fastener. When the collar is swaged onto the fastener shank portion, mating spiral grooves are formed along the interior surface of the collar. It is important to mention that in the existing fastener as well as in the one described by the above mentioned patents, the male fasteners (stud) is used in the installation process as a forming tool, intended to form the spiral groove into the collar. In order for the installation to be successful and meet all application requirements, the bolt needs to be manufactured from a material much harder than the collar material.
In order to remove the collar from the fastener shank portion, a tool having pivotal jaw members is positioned in the space formed between the bottom surface of the shank portion and the overhanging portion of the swaged collar, the jaw members being forced apart which, in turn, forces the collar to expand outwardly from the shank portion. During this expansion, the collar material is supposed to fracture, exposing the tail portion of the bolt. A tool then engages the collar, the collar being removed when the tool rotates, or spins, in the clockwise direction, pushed out by the helical groove formed in the collar by the male fastener at installation. The process of collar removal after it has been forced open, can be best described as xe2x80x9cthreading the collar outxe2x80x9d. The fastener disclosed in these patents provided a swaged collar to be quickly and easily removed while not damaging the gripping surfaces formed on the fastener shank portion, thus increasing the fastener reusability and decreasing the disassembly down time, in turn providing significant cost savings.
However, under certain performance criteria the prior art fastener described in the above mentioned U.S. patents was subject to problems. Specifically when the male fastener (stud) material was not hard enough, or was subjected to temperatures beyond the transformation point of the fastener material, as high as 2300xc2x0 F., the performance of the fastener could not meet the application requirements. Such applications exist when used, for example, in the exhaust systems for F-22 engines. The prior art fastener design was also highly susceptible to operator errors during installation, and the installation pre-load could not be controlled, causing rejections and excessive re-work. During the removal process, after being split open by a special tool, the collar needs to be pushed out of the stud, by using the spiral grooves formed inside of the collar during the installation process. In some cases, especially when the stud material is softened by exposure at high temperatures, the spiral groove is not formed well enough to give a positive axial force towards unthreading the collar, stripping before the collar is pushed out of the stud. In cases like this, the collar needs to be removed by other means, and in the removal process the stud may get damaged. Other problems with the prior art design included low or no re-usability because of the potential problems described previously, low or no pre-loads, faulty installations due to tool misalignment with the fastener during installation and a high standard of deviation of the load capability of the installed fastener.
Therefore, it is desired to provide an improved fastener which incorporates a threaded swageable collar which eliminates the problems associated with previous designs and is usable even when manufactured from softer, lighter materials, and in all conditions such as, for example, in temperatures beyond the transformation point.
As shown in FIGS. 1 and 2, the present invention provides an improved fastener comprising a male part of the system 12, named bolt or stud (depending on the application) having a head portion, a flange portion, a shank portion capable of extending through aligned apertures formed in adjacent panels, and a female part of the system 14, a threaded collar for receipt on the shank portion of the fastener. The shank portion of the male fastener presents a high strength thread 22 to provide a plurality of gripping surfaces having angled surfaces form between a plurality of spiral grooves. The female part of the system, named xe2x80x9ccollarxe2x80x9d has a plurality of mating internal threads 36 and is positioned over the end of the shank portion and threaded onto the shank until a certain installation torque is reached, accurately controlling the installation pre-load of the fastener. The end of the collar is then swaged over a locking groove 26 on the end of the shank to positively lock the collar 14 onto the shank of the stud 12.
The threaded portion 22 on the shank of the male part 12 of the fastening system, in combination with the internal threads 36 on the collar 14 provides the desired installation pre-load and the majority of the axial load capability. By comparison, on the previous art fastener, the load carrying capability was given by both the spiral groove and by the swaged portion at a ratio of almost 50% each. Because of this, in cases when the stud material was softened by exposure to very high temperatures, the installed system lost almost a half of the load capability. It also provides the axial force required during the removal of the collar. The length of the threaded portion 22 of the male part of the fastening system (stud or bolt) is established by the maximum designed grip range. The locking groove 26 on the end of the shank is designed to provide good material flow during swaging. A locking shoulder 28 is positioned behind the groove and is designed to provide maximum anti-rotational resistance of the collar after installation, as well as easy removal of the collar 14.
Likewise, as shown in FIG. 2, the length of the threaded portion 36 on the internal diameter of the collar is established by the load requirements and is constant for different grip ranges. The length of the swaging portion 38 of the collar 14 is directly dependent on the design maximum grip capability (thickness of the panels to be fastened) of the fastening system. The material thickness of the swaging portion 38 is computed such as to provide the desired anti-rotational, vibratory and axial load resistance as well as to provide good material flow. As shown in FIG. 3, located on the end 14A of the collar adjacent the swaging portion are two recesses or notches 40, used at installation to thread the collar until it meets the desired installation pre-load. This feature also aids in the removal process of the collar providing the expected re-usability of the male fastener (stud or bolt).
The fastening system of the present invention differs fundamentally from prior art in that prior fastener designs relied on the bolt to act as a mandrel, forming a spiral groove inside of the collar during the swaging process. This approach made the performance of the system very susceptible to hardness and dimensional variations making the installation on softer bolts unreliable or even impossible. The present invention does not use the stud 12 (or bolt) as a mandrel (or forming tool) during installation, so exposure to high temperatures during processing or in the application, and use of softer bolts has little effect on the performance and re-usability of the system.